Tropical forests are threatened by many types of farming, ranching, and human activity. One of the best ways to track those threats is through the Landsat series of satellites, developed by NASA and operated by the U.S. Geological Survey. The accessible and free archive of Landsat data makes it possible for scientists to monitor changes in remote landscapes without leaving their offices and labs.

Remote sensing scientist Clinton Jenkins of North Carolina State University had just such an opportunity earlier this year. In July 2013, he received a tip from colleagues in South America about some possible deforestation in the Loreto region of Peru. Jenkins and colleagues then began combing through recently acquired Landsat images for signs of change. Within hours, they found an image with what appeared to be bare ground in previously intact forest east of Tamshiyacu, Peru. But the image was cloudy, a problem for remote sensing of most tropical regions in daylight.

So the researchers waited. Every eight days, either Landsat 7 or Landsat 8 passed over the region, and every eight days, they got another cloudy image with hints but no confirmation of cleared forest. Finally on August 28, 2013, Landsat 8 got a clear view of the area. The top image above, from the Operational Land Imager on Landsat 8, shows the extent of the new deforestation. The lower image, from the Enhanced Thematic Mapper Plus on Landsat 7, shows the same area on October 5, 2012.

“Landsat imagery is essential for environmental monitoring because it is free, easy to access, and quickly available after the satellite passes over an area,” said Jenkins, who has been studying deforestation since he was a graduate student. “Satellites are the only way to monitor these areas because they are so large and so difficult to access.”

By piecing together evidence from multiple Landsat 7 and 8 images, Jenkins and colleagues have estimated a deforestation rate of roughly 100 hectares (247 acres) per week in the tract they observed. As of early September 2013, at least one thousand hectares were cleared near Tamshiyacu, according to Jenkins. Using a NASA-funded dataset on tropical biomass, he estimated that 300,000 tons of biomass were cut down, equal to 150,000 tons of carbon emitted to the atmosphere. Scientists are continuing to monitor the area by satellite, but cloud cover continues to be a problem.

Across Amazonia, as well as tropical Asia, one of the newest threats to forests is the clearing of land for palm oil plantations. The production of oil palm has been dominated by Indonesia and Malaysia, where vast tracts of palm trees have replaced much of the native forest in recent decades, often through burning. With such deforestation now escalating in the Amazon, scientists are concerned about the impact on biodiversity and on the planet’s carbon budget.

“Without the Landsat archive, there is simply no way we could have documented this deforestation fast enough to have any chance of stopping it,” Jenkins said. “Delayed access could mean thousands of hectares of forest destroyed, largely unseen, until it is too late.”

“Tracking forest disturbance, particularly tropical deforestation, is one of the landmark applications of Landsat data,” said Jim Irons, NASA’s project scientist for the Landsat program. “Through the archive and its open data policy, the evidence of forest disturbance is transparent, unbiased, conclusive, and available to all.”